The present invention relates to a method for cleaving arylethers.
This method is used in processes for preparing 4,6-diaminoresorcinol, a monomer used in preparing polybenzoxazoles (PBO). Although there are a number of known methods for preparing 4,6-diaminoresorcinol, there continues to be a need to find more efficient and cost effective routes to obtain 4,6-diaminoresorcinol.
One known method involves synthesizing the monomer from 1,2,3-trichlorobenzene as described in U.S. Pat. No. 4,766,244 issued to Lysenko. However, 1,2,3-trichlorobenzene has limited availability.
Another method for preparing 4,6-diamino-resorcinol involves treating 1,3-dichloro-4,6-dinitrobenzene with base to form 4,6-dinitroresorcinol. Although 4,6-dinitroresorcinol may be reduced to form 4,6-diaminoresorcinol, the product recovery is prohibitively low for commercial value.
In yet another method, the appropriate arylether such as di-arylmethoxy-dinitrobenzene can be cleaved to produce 4,6-diaminoresorcinol. U.S. Pat. No. 5,072,053, issued to Blank et al., describes cleaving arylethers by converting di-arylmethoxydinitrobenzenes to 4,6-diaminoresorcinol by catalytic reduction using a platinum metal supported catalyst, which cleaves the diethers and reduces the nitro groups to amines. However, this described method also produces toluene as an unwanted by-product which must be removed or converted back to benzyl alcohol for recycle.
Other known methods for cleaving arylethers are described in Protective Groups in Organic Chemistry by Theodora W. Greene, pp. 88-100, 1981, J. Wiley & Sons using, for example hydrobromic acid or hydroiodic acid. Unfortunately, large amounts of hydrobromic acid or hydroiodic acid are required. In addition, when hydroiodic acid is used to cleave dinitroarylethers, e.g., methyl and benzyldinitroarylethers, the reaction is complicated in that the amines formed from the reduction of the nitro groups with iodide are subsequently alkylated by the alkyl iodide present. This reaction produces unwanted by-products, particularly when the desired product is 4,6-diaminoresorcinol used in benzoxazole polymerization.
In yet another method, as described in an article by Bernard et. al., in Synthesis, Apr. 1989, pp. 287-289, alkyl arylethers are cleaved using lithium chloride in an N,N-dimethylformamide solvent. However, this method requires a three-fold excess of lithium chloride which gives added expense to the method.
Accordingly, it remains highly desirable to provide a method for cleaving arylethers which does not have the disadvantages of the prior art.